2 THE TRANSATLANTIC LONGITUDE. 



companies their ready promise of all needful facilities for determining the relative 

 longitude of their terminal stations. 



Immediately upon the landing of the cable at Trinity Bay, Mr. Hilgard was dis- 

 patched to this remote spot, in order to decide from personal inspection whether 

 the communication was sufficiently good to permit of satisfactory longitude-signals, 

 without delay ; but his report was necessarily adverse. 



Upon the organization of the telegraphic cable-expedition of 1865, Mr. Hilgard, 

 who during Prof Bache's illness was acting in his behalf, obtained anew from the 

 respective companies permission for the use of the cable, if successfully laid ; and 

 the Hon. Secretary of the Treasury authorized the necessary outlays. Mr. L. F. 

 Pourtales repaired to Heart's Content, and there awaited the arrival of the Great 

 Eastern,. in order to inform me without delay of the character and availability of 

 the signals, 'should the cable be successfully laid; but the rupture of the cable in 

 mid-ocean made his expedition unavailing. 



The same preliminary steps were again taken in 1866, Mr. G. W. Dean awaiting 

 the arrival of the Great Eastern at Heart's Content. The expedition of this year 

 was happily successful, and Mr. Dean reported by telegraph that the sharpness of 

 the signals was all that could be desired. Measures were at once taken for organ- 

 izing the parties. Mr. Dean returned only a few hours too late to present his 

 report while we were attending the session of the National Academy of Sciences 

 at Northampton; but he found Mr. Hilgard and myself at the meeting of the 

 American Association in Buffalo, where all the details for the expedition were 

 arranged without delay, and the needful directions for preparation of instruments 

 and observers given by Mr. Hilgard. 



The large interval between the meridians of the two extremities of the cable pre- 

 cluded the employment of the method of star-signals, for many reasons. This 

 method requires a more protracted occupation of the cable than it seemed right or 

 reasonable to solicit; the climate of Newfoundland, according to the best informa- 

 tion received, is too uncertain and variable to warrant reliance upon the continuance 

 of a clear sky for three hours, while unless the promise should be favorable it 

 would be unwise to employ the cable for transmitting observations from Valencia, 

 which would be useless unless combined with subsequent observ-ations of the same 

 stars from Heart's Content. Moreover, for a longitude so great as that to be mea- 

 sured, the special advantages of the method of star-signals chiefly disappear ; the 

 clock-rates becoming matters of serious importance, and entailing errors of the same 

 order of magnitude as those of the absolute time-determination, while the wide 

 separation of the observers precludes that thorough elimination and control of per- 

 sonal equation which is feasible Avhen the longitude-observations are restricted to 

 zenithal stars, and the observers can easily exchange positions and frequently meet 

 at one or the other station. 



There Avas also grovmd for confidence that the catalogue of standard stars to be 

 employed for determining time was so well freed from systematic errors, that the 

 difference of half a quadrant in the meridians would introduce no error depending 

 on the right-ascensions, no matter at what hour the comparison^ might be made — a 

 confidence which the event has fully justified. 



